Synthesis and small molecule chemistry of the niobaziridine-hydride functional group
Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Chemistry, 2005.
| Main Author: | |
|---|---|
| Other Authors: | |
| Format: | Thesis |
| Language: | eng |
| Published: |
Massachusetts Institute of Technology
2006
|
| Subjects: | |
| Online Access: | http://hdl.handle.net/1721.1/32479 |
| _version_ | 1826211721978576896 |
|---|---|
| author | Figueroa, Joshua S |
| author2 | Christopher C. Cummins. |
| author_facet | Christopher C. Cummins. Figueroa, Joshua S |
| author_sort | Figueroa, Joshua S |
| collection | MIT |
| description | Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Chemistry, 2005. |
| first_indexed | 2024-09-23T15:10:28Z |
| format | Thesis |
| id | mit-1721.1/32479 |
| institution | Massachusetts Institute of Technology |
| language | eng |
| last_indexed | 2024-09-23T15:10:28Z |
| publishDate | 2006 |
| publisher | Massachusetts Institute of Technology |
| record_format | dspace |
| spelling | mit-1721.1/324792019-04-10T12:12:23Z Synthesis and small molecule chemistry of the niobaziridine-hydride functional group Figueroa, Joshua S Christopher C. Cummins. Massachusetts Institute of Technology. Dept. of Chemistry. Massachusetts Institute of Technology. Dept. of Chemistry. Chemistry. Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Chemistry, 2005. Vita. Includes bibliographical references. Chapter 1. Synthesis and Divergent Reactivity of the Niobaziridine-Hydride Functional Group The synthesis, characterization and reactivity of the niobaziridine-hydride complex Nb(H)([eta]²-t- ]Bu(H)C=NAr)(N[Np]Ar)₂ (la-H; Np = neopentyl, Ar = 3,5-Me₂C₆H₃) is discussed. The niobaziridine-hydride functional group in complex la-H serves as a protecting group for the reactive three-coordinate d² species, Nb(N[Np]Ar)₃ (2a), via reversible C-H activation of the 3- ]H-containing N[Np]Ar ligand. At elevated temperatures, complex la-H rapidly converts to the neopentylimido complex, Nb(NNp)(Ar)(N[Np]Ar)₃ (3a), which is the product of N[Np]Ar ligand C'-N oxidative addition by putative three-coordinate 2a. Although not observed directly, the evidence for the intermediacy of 2a in N[Np]Ar ligand C-H and C-N activation processes has been obtained through isotopic labeling (H/D) studies. To further ascertain the propensity of la- H to serve as a masked form of 2a, its reactivity with small-molecule substrates was surveyed. Treatment of la-H with nitrous oxide (N₂0) or triphenylphosphine oxide (OPPh₃), readily generated the oxo Nb(V) complex, ONb(N[Np]Ar)₃, thus establishing la-H as a source of the potent two-electron reductant 2a. Complex la-H was also found to effect the two-electron reduction of a host of other inorganic substrates. However, when treated with certain unsaturated organic molecules, insertion into the Nb-H moiety of complex la-H is observed in which the niobaziridine ring is left intact. Based on synthetic studies, a coordinatively induced, C-H bond reductive elimination mechanism is proposed for reactions between la-H and small molecules. (cont.) This mechanistic proposal accounts for both the observed insertion and two-electron reduction behavior exhibited by niobaziridine-hydride la-H. To extend the generality of niobaziridine- hydride functional group as a protecting group for three-coordinate Nb(NR₂)₃ species, the complexes Nb(H)([eta]²-Me₂C=NAr)(N[i-Pr]Ar)₂ (lb-H) and Nb(H)([eta]²-Ad(H)C=NAr)(N[CH₂- Ad]Ar)₂ (1c-H) were synthesized. The thermal behavior of niobaziridine-hydrides lb-H and lc- 1-I is compared and contrasted to that of the N-neopentyl variant la-H. Chapter 2. Activation of Elemental Phosphorus: Synthesis of an Anionic Terminal Phosphide of Niobium The niobazinidine-hydride complex Nb(H)( [eta]²-t-Bu(H)C=NAr)(N[Np]Ar)₂ (la-H; Np neopentyl, Ar = 3,5-Me₂C₆H₃) was found to react quantitatively with elemental phosphorus (P₄) to provide the bridging diphosphide complex ([mu]₂:[eta]²,[eta]²-P₂)[Nb(N[Np]Ar)₃]₂ ([mu]-P₂)[2a]₂. Reductive cleavage of ([mu]-P₂)[2a]₂ with sodium amalgam afforded the sodium salt of the terminal niobium phosphide anion, [PNb(N[Np]Ar)₃)]⁻ ([2a-P]⁻), which is best formulated as containing a Nb-P triple bond. The phosphorus atom of [2a-P]⁻ has proven to be nucleophilic and is readily functionalized upon addition of an electrophile. Treatment of [2a-P]⁻ with trimethylstannyl chloride provided the terminal phosphinidene complex Me₃SnP=Nb(N[Np]Ar)₃ which contains a P-Sn single bond. However, treatment of [2a-P]⁻ with ClP(t-Bu)₂ or ClP(Ph)₂ provided niobium- complexed [eta]²-phosphinophosphinidene complexes, which contain considerable P-P multiple bonding character. Thus, substantial electronic reorganization of the Nb [equal] P moiety in [2a-P]⁻ is induced upon functionalization. (cont.) The tendency for the Nb[equal]P unit in [2a-P]- to undergo electronic reorganization has been exploited, resulting in the synthesis of a complexed [eta]²-P,P- diphosphaorganoazide (PPNR) species, which eliminates a 'P2' unit when heated. Furthermore, treatment of the phosphido anion [2a-P]⁻ with divalent group 14 salts affords complexes of the formulation ([mu]₂:[eta]₃,[eta]₃ -cyclo-EP₂)[Nb(N[Np]Ar)₃]₂ (E = Ge, Sn, Pb). The bridging cyclo-EP₂ units in these complexes can be considered as neutral 2[pi]-electron, three-membered rings isolobal to the cyclopropenium ion. The molecular and electronic structure of anion [2a-P]⁻ and several of its derivatives are discussed. Chapter 3. Isovalent Pnictogen for O(Cl) Exchange Mediated by Terminal Pnictide Anions of Niobium Reported herein is a new, metathetical P for O(Cl) exchange mediated by an anionic niobium phosphide complex which furnished phosphaalkynes (RC [equal] P) from acyl chlorides (RC(O)Cl) under mild conditions. The niobaziridine hydride complex, Nb(H)(t-Bu(H)C=NAr)(N[Np]Ar)₂ (la-H, Np = neopentyl, Ar = 3,5-Me₂C₆H₃), has been shown in chapter 2 to react with elemental phosphorus (P₄) affording the [mu]-diphosphide complex, ([mu]²:[eta]²,[eta]²-P₂)[Nb(N[Np]Ar)₃]₂ (([mu]- P)[2a]₂), which can be subsequently reduced by sodium amalgam to the anionic, terminal phosphide complex, [Na][PNb(N[Np]Ar)₃] (Na[2a-P]). (cont.) Treatment of Na[2a-P] with either ]pivaloyl (t-BuC(O)Cl) or 1-adamantoyl (1-AdC(O)Cl) chloride provides the thermally unstable, niobacycles, (t-BuC(O)P)Nb(N[Np]Ar)₃ (2-t-Bu) and (1-AdC(O)P)Nb(N[Np]Ar)₂ (2-1-Ad) which are intermediates along the pathway to ejection of the known phosphaalkynes t-BuC-P (3- t-Bu) and 1-AdC [equal] P (3-1-Ad). Phosphaalkyne ejection from 2-t-Bu and 2-1-Ad proceeds with formation of the niobium(V) oxo complex ONb(N[Np]Ar)₃ (2a-0) as a stable byproduct. Preliminary kinetic measurements for fragmentation of 2-t-Bu to 3-t-Bu and 2a-0 in C₆D₆ solution are consistent with a first order process. Separation of volatile 3-t-Bu from 2a-0 after thermolysis has been readily achieved by vacuum transfer in yields of 90%. Pure 2a-0 is recovered after vacuum transfer and can be treated with 1.0 equivalent of triflic anhydride (Tf₂O, Tf = SO₂CF₃) to afford the bistriflate complex, Nb(OTf)₂(N[Np]Ar)₃ (2a-(OTf)₂), in high yield. Complex 2a-(OT'f)₂ provides direct access to la-H upon reduction with magnesium anthracene, thus completing a cycle of element activation, small-molecule generation via metathetical P-atom transfer and deoxygenative recycling of the final niobium(V) oxo product. Extension of this metathetical P for O(C1) exchange to the synthesis of novel phosphaalkynes is discussed. In addition, the analogous N for O(C1) exchange reaction for the synthesis of organonitriles from the niobium nitrido anion, [NNb(N[Np]Ar)₃]⁻ ([2a-N]⁻) has been developed. Nitrido anion [2a- N]⁻ is obtained in a heterodinuclear N₂ scission reaction which employs the molybdenum trisamide system as a reaction partner. (cont.) Treatment of [2a-N]⁻ with acyl chloride substrates rapidly furnishes organonitriles concomitant with the formation of niobium oxo 2a-0. Deoxgenative recycling of 2a-0 to a niobium complex appropriate for heterodinuclear N2 scission has been developed as well. Utilization of ¹⁵N-labeled ¹⁵N₂ gas in this chemistry has afforded a series of ¹⁵N-labeled organonitriles which have been characterized by solution ¹⁵N NMR. While, no intermediate complexes are observed during the organonitrile formation process, synthetic and computational studies on model systems provide strong evidence for the intermediacy of niobacyclic species. by Joshua S. Figueroa. Ph.D. 2006-03-29T18:48:16Z 2006-03-29T18:48:16Z 2005 2005 Thesis http://hdl.handle.net/1721.1/32479 61772226 eng M.I.T. theses are protected by copyright. They may be viewed from this source for any purpose, but reproduction or distribution in any format is prohibited without written permission. See provided URL for inquiries about permission. http://dspace.mit.edu/handle/1721.1/7582 161 p. 11979256 bytes 11988188 bytes application/pdf application/pdf application/pdf Massachusetts Institute of Technology |
| spellingShingle | Chemistry. Figueroa, Joshua S Synthesis and small molecule chemistry of the niobaziridine-hydride functional group |
| title | Synthesis and small molecule chemistry of the niobaziridine-hydride functional group |
| title_full | Synthesis and small molecule chemistry of the niobaziridine-hydride functional group |
| title_fullStr | Synthesis and small molecule chemistry of the niobaziridine-hydride functional group |
| title_full_unstemmed | Synthesis and small molecule chemistry of the niobaziridine-hydride functional group |
| title_short | Synthesis and small molecule chemistry of the niobaziridine-hydride functional group |
| title_sort | synthesis and small molecule chemistry of the niobaziridine hydride functional group |
| topic | Chemistry. |
| url | http://hdl.handle.net/1721.1/32479 |
| work_keys_str_mv | AT figueroajoshuas synthesisandsmallmoleculechemistryoftheniobaziridinehydridefunctionalgroup |